The half-lives of chloroethyl isocyanate and cyclohexyl isocyanate have been determined. Based on the half-lives it is possible to calculate the isocyanate concentration present during exposure of cells to BCNU and CCNU. Nitrosourea concentrations which are highly toxic to L1210 cells generate isocyanate levels which show little toxicity. BIBLIOGRAPHIC REFERENCES: Hilton, J. and Walker, M.D. DNA strand Scission and Its Repair Following Exposure of Cells to Inhibitors of Oxidative Phosphorylation. Biochem. Biophys. Res. Commun., 75: 909-914, 1977. Hilton, J., Bowie, D.L., Gutin, P.H., Zilo, D.M. and Walker, M.D. DNA Damage and Repair in L1210 Cells Exposed to CCNU. Cancer Research, in press, 1977.